It will be appreciated by those skilled in the art that traffic may come in many different forms. For example, considering solely traffic on land, such traffic can take a variety of forms, including but not limited to vehicles on roads, bicycles on paths, trains on rails, people on paths, aircraft on runways, etc.
There are several reasons why information regarding traffic on a particular section of a traffic route (for example a road, a path, a railway line, etc) may be collected. One of these may be for the effective management of traffic, where information regarding the speed and volume of traffic is useful. This enables alternative routes to be planned in response to accidents or route closures and to attempt to relieve congestion, perhaps by altering speed limits.
Considering the example of roads, many new roads are built with a sacrificial top layer which is designed to wear out and be replaced. The significant costs associated with road repairs and road building, in addition to the disruption caused by such works, requires that repairs are carried out only when needed. The sacrificial layer should neither be replaced too soon, leading to unnecessary costs, nor too late, risking more serious damage to the underlying structure of the road. An accurate determination of the volume of traffic on a particular road section is therefore essential.
A further reason why traffic information is required is for the enforcement of regulations and laws. There are regulations relating to maximum allowable weights for heavy goods vehicles (HGVs) which are borne out of concerns for safety and also to lessen the damage that overladen vehicles may do to the road structure. A measure of dynamic vehicle weight helps to ensure that such regulations are adhered to. This also applies to other forms of traffic route, for example to the issue of overweight trains on particular railway routes. Particular lines may be approved for use by trains up to a certain weight maximum, and the rail operators should adhere to these weight restrictions. Again, the ability to measure dynamic weight would help to ensure that such regulations are adhered to.
Simple information regarding vehicle speed may be used to monitor and enforce speed limits.
There may also be a requirement to collect information regarding the types of vehicle using a particular section of road. This may be to prevent unsuitable vehicles such as HGVs from using rural roads or to plan future road building schemes. Classification of vehicle type may be achieved from a determination of dynamic vehicle weight and axle count.
It is clear that information regarding the speed, weight, volume and type of traffic can also be used to help with an effective traffic management programme. There are several methods in use to obtain this information, however these have associated problems.
Many sections of road are overseen by video cameras. The images from these cameras are fed to central points to be analysed to provide information regarding vehicle speed and type and traffic volume. However, due to the complexity of the images, it is not always possible to reliably automate the analysis of the data received, meaning that they must be studied visually. There is a limit to how many images can be analysed in this way. Furthermore, the quality of the images collected may be influenced by weather conditions. Fog or rain can obscure the field of view of the cameras, as can high vehicles, and high winds can cause the cameras to vibrate. In many countries, camera systems are operated by law enforcement agencies, so there is often an added complication in making the information collected available to the agencies involved with traffic management. It is also not possible to determine the weight of a vehicle from a video image. The commissioning costs of video camera systems for traffic monitoring can also be high.
The vast majority of new roads and large numbers of existing roads are provided with inductive sensors. These are wire loops which are placed below the road surface. As a vehicle passes over the sensor, the metal parts of the vehicle, i.e. the engine and the chassis, change the frequency of a tuned circuit of which the loop is an integral part. This signal change can be detected and interpreted to give a measure of the length of a passing vehicle. By placing two loops in close proximity to one another, it is also possible to determine the vehicle's speed. The quality of the data collected by inductive loop sensors is not always high and is further compromised by the fact that the trend in many modern vehicles is to have fewer metal parts. This leads to a smaller signal change which is more difficult to interpret. In addition, because of a tendency for road builders to make greater use of steel in highway construction, there is an increasing problem of interference affecting the accuracy of readings.
Although cheap to produce, inductive sensors are large and as such their placement, particularly in existing roads, causes significant disruption. This has associated costs. A major drawback with the use of inductive loops for traffic management is that they are not amenable to multiplexing. Each sensor site requires its own data collection system, power supply and data communication unit. This increases the cost of the complete sensor significantly, which results in the majority of installed inductive loops not being connected, and therefore incapable of collecting data. Furthermore, although inductive loops can be used to count vehicles and, if deployed in pairs, to determine vehicle speed, they cannot be used to measure dynamic vehicle weight. Vehicle classification is thus not possible.
Two methods for determining the weight of vehicles, in particular HGVs are in common use. Vehicle weight can be measured using a weigh-bridge. This is very accurate but requires the vehicle to leave the highway to a specific location where the measurement can take place. An alternative method is to attempt to measure the weight of the vehicle as it is in transit. Commonly, piezo-electric cables are placed under the surface of the road, which produce a signal proportional to the weight of the vehicle as it passes over. This method is more convenient but less accurate than a weigh-bridge. As with inductive loop sensors, piezo-electric sensors are not amenable to multiplexing so each requires a similar data collection system, power supply and data communication unit. The sensors are also more expensive and less robust than inductive loop sensors.
In order to obtain the maximum amount of information regarding traffic on a particular section of road, piezo-electric sensors are often deployed in tandem with inductive loops.
Optical fibre sensors can be used to detect pressure. When a length of optical fibre is subjected to an external pressure the fibre is subjected to a strain. This strain imposes a change in property (e.g. phase) in an optical signal propagating through the optical fibre due to a combination of the physical length change and the stress-optic effect, and this change in property can be detected. As it is possible to analyse for very small changes in property such as phase, optical fibre sensors are extremely sensitive to applied pressure. This high sensitivity allows optical fibre sensors to be used, for example, in acoustic hydrophones where sound waves with intensities equivalent to a pressure of 10−4 Pa are routinely detectable. Such high sensitivity can however also cause problems. Optical fibre sensors are not ideally suited for use in applications where a low sensitivity is required, for example for detecting gross pressure differences in an environment with high background noise. However, optical fibre sensors have the advantage that they can be multiplexed without recourse to local electronics.
It would be desirable to provide a sensor for traffic monitoring which would be easily deployed, would provide the required accuracy and could be multiplexed with other sensors to simplify data collection, data communication and power supply.